Some wireless telemetry systems (e.g., burglar alarms, fire alarms, power utility meters, leak detectors, environmental monitoring, plant control, etc.) comprise many transmitters that intermittently (periodically or sporadically, at constant or varying time intervals) transmit messages to one or more receivers. In these systems, the transmitters are located at different places and transmit messages that are indicative of the status of monitoring sensors to a receiver that collects the data from all of the sensors. Normally, the transmitters transmit messages that are as short as feasible and with the interval between the transmissions as long as feasible. This is advantageous for two reasons. First, it minimizes the average current drain in the transmitters, which are typically battery operated. Second, short and infrequent transmissions lower the probability that the data is lost due to collisions that occur when two or more transmitters transmit at the same time.
In order that a receiver can discern which data comes from which transmitter, each transmitter has a transmitter identification number assigned to it. The transmitter identification number is included in each transmitted message in order to make it possible for the receiver to identify the source of each received message.
Preferably, the transmitter identification number is determined for each transmitter as a large and unique number in order to ensure uniqueness when many systems operate in vicinity such that one receiver can receive messages from transmitters from more than one system.
In order to avoid a complex and expensive management of the transmitter numbering depending on the geographical location of the installed systems to prevent such confusion, it is preferred to equip each manufactured transmitter with a unique number that is never repeated. Such a number can be very large (depending on the predicted total number of transmitters ever to be produced) and may require a large number of bits in the transmitted messages. In effect, a significant overhead in the transmission is created, which increases the transmitter average power consumption. In battery operated transmitters, this overhead shortens the life of the transmitter battery. In some transmitters the battery life may be shortened by half or even more, thus increasing system maintenance cost and lowering system functionality and reliability.
Typically, such systems transmit data at a single frequency, and thus are susceptible to interference and signal loss due to phenomena known as “multipath fading”. Consequently, the reliability of such systems is compromised or, conversely, the transmitted power has to be increased to overcome the fading, which results in larger power drain and shorter battery life. Besides, there usually are regulatory limits that restrict such transmitter power and thus limit the possible compensation by sheer increase of power. Since the multipath effect is highly sensitive to the frequency of the transmitted carrier, a system using multiple frequencies (e.g., a frequency hopping spread spectrum system, etc.) has a potential to eliminate these drawbacks. However, frequency hopping systems require a long acquisition time and they are typically used in two way communication applications in which all the devices are continuously synchronizing with one master device or with each other using a variety of synchronization methods as shown in some references. In other cases, to ease the synchronization problem, there are employed receivers that can simultaneously receive signals at many frequencies by making the receiver broadband or by using several receivers at the same time. Generally, those solutions suffer from performance degradation or high cost or both, which makes them undesirable for low cost applications that require high reliability such as security systems and many other telemetry systems.